Hackers have reportedly broken into a computer system at CERN's Large Hadron Collider, targeting a system that was "one step away" from a control computer, but otherwise appear to have done no major damage.

The system that was breached monitors the Compact Muon Solenoid Experiment, which will be analysing data during subatomic particle collisions in the particle accelerator located along the French-Swiss border. Experiments, which began on Wednesday, are designed to help scientists explore particle physics theories.

During the attack on Tuesday and Wednesday, according to a report on Friday in the British newspaper The Telegraph, hackers left behind half a dozen files, damaged one CERN file, and displayed a web page with the headline "GST: Greek Security Team," signing off: "We are 2600: don't mess with us," CERN scientists told the newspaper.

As a result of the attack, the web site, cmsmon.cern.ch was not accessible late last week.

Fears that the experiments could prompt man-made disasters or black holes that would swallow Earth have led to threatening phone calls and emails, CERN said.